This invention relates to equipment for economically and effectively cleaning and chemically sanitizing food-handling articles at a high rate of productivity. An example of such equipment is a dishwasher for use in commercial applications like restaurants, cafeterias, hospitals and other institutions where dishes are frequently reused during a meal period, although the sanitizing principle is applicable to any kind of ware with which food comes into contact.
It is necessary in the use of such equipment to destroy bacteria during a rinsing operation to meet minimum sanitation standards. That is generally done by providing high temperature rinse water, e.g., 180.degree.-195.degree. F., or, where such temperatures are not achievable, by adding a chemical sanitizing agent to low temperature water (approximately 120.degree.-140.degree. F.) to produce the bacteria-killing effect. Use of the terms "high" and "low" herein relate approximately to the above temperature ranges. The present invention is concerned with the latter approach of cleaning and sanitizing food-handling articles with low temperature water and a chemical additive, such as is illustrated in U.S. Pat. Nos. 2,592,884, 2,592,885, 2,592,886, 3,044,092, 3,146,718, and 3,370,597, all of which are assigned to the assignee of the present invention.
Although several different types of chemical sanitizing agents are available on the market, the one most commonly used today is liquid sodium hypochlorite (NaOCl), because of its high degree of effectiveness, relatively low cost, and general availability. This particular chemical, however, is not without its disadvantages, the most common being its chemical reaction with hard water minerals like iron, calcium, and magnesium, the latter two causing liming or mineral deposits onto the machine parts with which they come in contact. These deposits also tend to build upon orifices when a water powered venturi is used to draw the agent from a supply thereof into the water line en route to the rinse nozzles of the washing machine. The deposits continue to change the proportion of NaOCl to a given volume of water as they build up on the venturi. Eventually the volume of NaOCl becomes insufficient for santizing. Deposits also tend to clog the rinse nozzles themselves, often requiring frequent removal and cleaning to maintain their efficiency. For these reasons, devices such as disclosed in the aforementioned patents have limited reliability and have found limited application, both where the agent is injected into a rinse line or directly into a wash chamber. High temperature sanitizing equipment has achieved much greater use, even though the higher temperature requires considerable energy usage as well as higher initial cost for electric or gas-fired booster heater units.
In recent years, due to increasing prices and decreasing availability of energy, increased emphasis has been placed on chemical sanitizing warewashers to reduce energy consumption, and manufacturers are again introducing specialized equipment of this type to meet this need. However, to minimize the pressure variation and liming problems inherent in water introduction of sodium hypochlorite, and to maintain the efficiency and proper operation of their systems, users of systems which introduce the chemical sanitizing agent directly into a fresh water line have had to accept the need for frequency service calls from their chemical suppliers.
Possibly because of the flow pressure, mineral deposit, and frequent service problems associated with introducing a chemical sanitizing agent directly into a fresh water line of a dedicated rinse system, several U.S. manufacturers have also introduced chemical low temperature sanitizing dishwashers which operate essentially on the recirculating rinse principles described in U.S. Pat. No. 3,903,909. (However, not all of them interconnect the fresh water line and recirculating system as does the design of the '909 patent). The '909 device still uses a water driven venturi, but since the rinse fluids are mixed in the sump and recirculated, it is not necessary to maintain precise metering of the sanitizing solution into the water line, so long as the proper total amount is eventually injected.
Typically, such machines provide a wash chamber having a sump for containing wash water and a pump which draws water from the sump and recirculates it under pressure through nozzles in one or more rotating wash arms to spray the dishes. The wash water is drained from the sump after washing a load of dishes and is replaced by fresh rinse water. The rinse water, into which the sanitizing chemical is injected, is then sprayed and recirculated onto the dishes through the same pump and wash arms to provide a single, recirculated rinse. The rack containing the washed and rinsed dishes is then removed from the machine and replaced by a rack of dirty dishes. The rinse water is retained in the sump after rinsing, detergent is added thereto, and it is then used as the wash water for the next rack of dirty dishes. Ordinarily, these dirty dishes will have been scraped only, and thus contain gravies, residue of mashed potatoes, bread crumbs, small bits of food, etc.
Because the wash water must be drained after each wash in this type of machine, the sump, pump, and spray nozzles of the combined wash and rinse system are designed to operate with a minimum quantity of water, for example, as little as two gallons for each rack. This places an operational restriction on the pump, limiting its ability to deliver large volumes of water to the dishes in a short period of time, and forcing restrictions on the size of the openings in the wash and rinse nozzles, thus increasing their chances of clogging with food particles, Additionally, since large food particles frequently accidentally remain on the tops and bottoms of dishes when placed in the wash chamber, a strainer system is required to capture these larger particles and prevent their passing through the pump and clogging the nozzles. The strainers are generally provided with very closely spaced holes of 1/8 inch diameter or less, and are said to be 1/32 inch in the aforementioned '909 patent. What happens when using systems of this type, therefore, is that the smaller food particles and other tiny suspended granular objects pass through the strainer, and the pump continually redeposits them on the ware and on the inside surfaces of the wash chamber, the pump, the wash arms, and so on. Compromise is therefore necessary in designing the size of the strainer holes in order to satisfy conflicting conditions. On the one hand, the holes should be as small as possible to prevent passage of soil particles; on the other, they must be large enough to prevent strainer clogging and pump starvation with accompanying loss of water circulation. For this reason, redeposition of small soil particles in such machines is an unavoidable condition during washing.
The aforementioned '909 patent proposes to rinse out the spray arms and drain some of the rinse water before closing the drain by connecting the wash arms to both the fresh water line and the recirculating pump (with a check valve therebetween). Before the drain closes, approximately 20% of the water consumed in each cycle is immediately drained in an attempt to flush debris from the wash system and chamber. This water is lost, passing down the drain with the soiled wash water. Effective cleaning with such a system is still believed difficult, however, because of other operational compromises inherent in such a machine. For example, the commercial machines of a U.S. manufacturer believed to be the owner of the aforementioned '909 patent also have the drain maintained open while introducing approximately two quarts of fresh water for flushing purposes, but the water is introduced directly into the sump rather than into the wash arms. The structural design appears such that the pump probably cannot pick up much, if any at all, of this small quantity of water while the drain is open, and therefore cannot recirculate it for flushing the arms or the wash chamber. Some soil will therefore inevitably remain in the system.
On general principles as well, such retained soil is all but impossible to remove in the single rinsing action with the limited water volume which is commonly provided in commercial dishwashers of this particular design. The strainers conventionally found in these machines are designed in the form of baskets or trays which capture the larger food particles, to enable their easy lifting from the machine and dumping into a disposer or garbage pail. In order for the strainer to be effective, the recirculating water must pass continually through the strainer on the way to the pump intake, and therefore through the garbage in the strainer as well. The manufacturers therefore recommend frequent cleaning of these trays, to reduce the amount of soil which the recirculating rinse water must necessarily pass through. However, machine operators cannot be relied on to perform such tasks, particularly where more than one individual may use or be responsible for the machine during the same meal period. The end result in such machines is that, while the bacteria on the dishes may be properly killed, there is nevertheless a continual redeposition of fine soil even during rinsing. These effects--fine soil remaining in the recirculating system for the rinse, and soil remaining in the strainer--sometimes result in an unappetizing appearance or feel of the dishes, giving the user of the dishes the impression that they are unsanitary, even though the bacteria may have been destroyed.
As suggested above, the design direction in recent years for equipment for cleaning and chemically sanitizing dishes (as exemplified by the aforementioned U.S. Pat. No. 3,903,909), has also created a substantial reduction in productivity as compared to standard dishwashing machines utilizing high temperature water for sanitizing the dishes. In standard, high temperature dishwashers, the rinse water is normally introduced through a "dedicated" rinse system, i.e., one which is separate from the wash system and carries only fresh, very hot rinse water. Generally, the wash water in the wash system is saved in the wash system sump and reused for washing successive racks of dishes. Used rinse water from the independent rinse water system is conducted to the wash system, causing overflow of some of the used wash water through a standpipe connected to the drain, and continually replenishing the wash water supply with hot clean water. The rinse may use about two gallons (of which some will overflow through the standpipe before mixing with the wash water, so that only a part of the rinse water will dilute the wash water). Detergent is then added (usually automatically) to the wash water periodically because of this partial dilution.
Since the wash water is maintained in the sump rather than being drained each cycle, the volume thereof may be relatively large. This provides considerable flexibility in the design of the water pump and the size of the nozzle orifices in the wash arms, simply because the large volume of water in the sump permits usage of a high capacity pump for delivering water in large volumes through the spray system to the dishes.
Minimum total spray volumes are specified by the organizations that create industry standards. For example, Standard No. 3, Section 6.05 of the National Sanitation Foundation of the U.S. (N.S.F.), pertaining to Single Tank, Stationary-Rack, Door-Type Chemical Sanitizing Machines, requires not less than 80 gallons of water to be delivered for each 20 inch.times.20 inch rack for the combined washing and rinsing of a rack of dishes. The minimum pump delivery capacity is required to be at least 40 gallons per minute (g.p.m.). This is easy to achieve if the sump is large and plenty of water is available. However, if the sump is small and a minimum-capacity pump is used to deliver 80 gallons of water, the pump time during the cycle will be a full two minutes. This sets a theoretical production maximum of 30 racks of dishes per hour for a minimum capacity pump, but is achievable only if absolutely no time is required for water fill, drain, loading and unloading the racks, an impossibility even in an automated machine. Even if a high capacity pump is used, wash volumes and delivery rates will still be restricted far more in a machine of the '909 type than in one with a dedicated rinse system.
More particularly, these restrictions and this reduction in productivity are a result of several things. First, since the same sump and same spraying system are used for both washing and rinsing, it becomes essential to drain the sump for each machine cycle, i.e., for each rack of dishes washed, and this results in a loss of productive time. The machine must stop in the middle of each cycle and drain the sump almost completely, and sometimes flush as well, before the actual dish rinsing can commence. This waiting period is a minimum of 10 seconds, and may be as much as 30 seconds, before the rinse spray becomes effective, depending on how rapidly the sump fills. This seemingly small time actually constitutes a minimum of 8%, and as much as 25%, of a total two minute cycle time. When multiplied over a large volume of dishes, this could be a very serious cost disadvantage in labor alone.
Secondly, because of the cost of heating water even to the "low" temperature of 140.degree. F., the machines are constructed (as indicated above) to use as small a quantity of water as possible, approximately two gallons, plus a few extra quarts where a "flush" period is used. The amount of water thus consumed each cycle is only slightly greater than that used for rinsing in high temperature machines which sanitize by means of heat. However, since this rinse water (which is dumped during the next cycle) is the only water available in the sump for recirculation, the sump and pump capacities must be kept small. Obviously, the higher the pump capacity, the greater the supply of water that is required to feed the pump is order to prevent cavitation and attendant loss of pressure, which result in reduced effectiveness of the water spray contacting the dishes. Compensation is therefore made for the smaller quantity of water in the sump by reducing the pump capacity and restricting the orifice size of the nozzles of the spraying system. This in turn reduces the flow of water through the nozzles and reduces the volume of water which, in a given time period, contacts the dishes which are being washed. It also increases the chances that a nozzle will clog with particles of food and other materials. The reduced flow is then compensated for by extending the washing time, but this further reduces productivity.
The low temperature chemical sanitizing dishwashers such as illustrated in U.S. Pat. No. 3,903,909 and its commercial couterparts ordinarily lack a tank heater. They rely solely on the rinse water temperature to maintain adequate wash water temperature. The N.S.F. minimum temperature for washing is 120.degree. F. This requires the inlet fresh rinse water temperature to be about 140.degree. F., because the water cools as it is circulated by the pump and contacts the dishes and the walls of the dishwasher. Under some circumstances, such as in nursing homes, the water heater temperature may be around 120.degree. F. to begin with, requiring a separate booster heater for the fresh water line connected to the dishwasher. If after the rinse is completed, the next washing cycle is not started within a short time, the wash water will cool below the 120.degree. F. washing temperature. Under these operating conditions the machine must be cycled to bring in hot wash water to meet code requirements and to control foam and pump cavitation.
Thus, when considering the total costs of detergent, sanitizing chemical, rinse agent, heat energy for the water, machine depreciation and maintenance, and increased manual labor for each rack of dishes (due to reduced machine capacity), it is likely that the total cost to the user is greater when using a single rack machine such as illustrated in the '909 patent, than when using a comparable machine in which high-temperature sanitizing is employed. Labor alone is one of the major cost factors in washing dishes, and this is considerably reduced with the present apparatus and method as compared to that of '909 patent.
In addition to the standard high temperature dishwashers previously described, there are known to exist in other countries, particularly where hot water heaters are not readily available or are available only at low temperatures (perhaps 120.degree. F.), prior art dishwashers in which a low temperature fresh water supply line introduces water into a holding tank mounted on the dishwasher. The holding tank includes an air gap for physically separating the fresh water line and the water system of the dishwasher. The level of the water in the holding tank is controlled by a float which opens a valve in the fresh water supply line upon descent of the float, and closes the valve when the float reaches its upper level. Between the holding tank and rinse nozzles, which are dedicated solely to the rinse system, there is an auxiliary booster heater tank having heating coils for raising the low temperature water to the high temperature necessary to destory bacteria when rinsing. When rinsing is to take place, the recirculating pump for the wash water stops and an auxiliary pump in a water line between the holding tank and the auxiliary booster heater tank is operated to pump rinse water through the rinse nozzles. Fresh water is introduced into the holding tank as soon as the float begins to descend, functioning merely to maintain a supply of rinse water available for the rinse system. Control of the quantity of water utilized for rinsing is a function of the time the rinse pump operates.
Thus recently introduced machines for achieving chemical sanitization of dishes, while solving one problem, namely a reduction in consumption of energy by eliminating the need to heat water to 180.degree. F. or more, have thus introduced new problems in productivity, cost of operation, and poorer washing results, as compared with existing high temperature machines. The present invention proposes to solve the washability, productivity, and cost problems inherent in these prior art designs.